[Django]-Django Admin Form for Many to many relationship

I know that this is an older thread, but this was the first result that came up on google and I thought a better answer was necessary.

Via this django bug report I found the best way to have your ManyToManyField show up on both models:

class Test1(models.Model):
    tests2 = models.ManyToManyField('Test2', blank=True)

class Test2(models.Model):
    tests1 = models.ManyToManyField(Test1, through=Test1.tests2.through, blank=True)

I have tested it myself and was very pleased with the results.

The only built-in way is via an InlineModelAdmin, but you can use a custom ModelForm with your User ModelAdmin to create field for this purpose. See the code below for simplified setup (assumes users = ManyToManyField(related_name='domains')).

### yourapp/admin.py ###

from django import forms
from django.contrib import admin
from django.contrib.auth.models import User
from django.utils.translation import ugettext_lazy as _
from django.contrib.admin.widgets import FilteredSelectMultiple

from .models import Domain

class DomainAdmin(admin.ModelAdmin):
    filter_horizonal = ('users',)

class UserAdminForm(forms.ModelForm):
    domains = forms.ModelMultipleChoiceField(
        queryset=Domain.objects.all(), 
        required=False,
        widget=FilteredSelectMultiple(
            verbose_name=_('Domains'),
            is_stacked=False
        )
    )

    class Meta:
        model = User

    def __init__(self, *args, **kwargs):
        super(UserAdminForm, self).__init__(*args, **kwargs)

        if self.instance:
          self.fields['domains'].initial = self.instance.domains.all()

    def save(self, commit=True):
        user = super(UserAdminForm, self).save(commit=False)

        user.domains = self.cleaned_data['domains']

        if commit:
            user.save()
            user.save_m2m()

        return user

class UserAdmin(admin.ModelAdmin):
    form = UserAdminForm

admin.site.register(Domain, DomainAdmin)
admin.site.unregister(User)
admin.site.register(User, UserAdmin)

I think what you’re looking for is InlineModelAdmin.

For people who still bump into this, it might be worth checking ​https://github.com/kux/django-admin-extend

It provides a mechanism for injecting bidirectional many-to-many fields in ModelAdmins that have already been defined by other apps.